South Korea has formally decided to obtain 40 Lockheed Martin F-35As to fill its long running F-X III requirement.

“We are honoured by and appreciate the trust and confidence the Republic of Korea has placed in the 5th Generation F-35 to meet its demanding security requirements,” says Orlando Carvalho, executive vice president of Lockheed Martin Aeronautics.

“We look forward to supporting the discussions between the Republic of Korea and U.S. governments in support of a final agreement this year. This decision strengthens and extends our long-standing security partnership while enhancing regional stability across the greater Asia Pacific theatre.”

Lockheed’s statement follows a formal announcement by South Korea’s Defense Acquisition Program Executive Committee in Seoul earlier today.

If the acquisition proceeds as planned, deliveries will start in 2018.

A source familiar with South Korean defence acquisitions says Seoul will now issue a formal letter of request to the US for the aircraft and other elements of the programme, such as offset arrangements associated with the deal.

Media reports from South Korea indicate that the country hopes to conclude negotiations for the fighters by the third quarter of 2014.

Seoul’s decision follows a November 2013 report carried by South Korea’s official news agency quoting the nation’s joint chiefs of staff as saying that Seoul would buy 40 F-35As, with deliveries to start in 2018.

That report mentioned that Seoul would also obtain an option for an additional 20 F-35As, but today’s announcements from South Korea and Lockheed make no mention of this.

After Australia and Japan, South Korea will be the third Asia Pacific nation to order the type. Australia, a partner in the F-35 programme, could obtain up to 100 F-35As. Japan is obtaining 48 aircraft under the US Foreign Military Sales mechanism, through which Seoul will also acquire its aircraft.

Among other regional powers, Singapore has expressed strong interest in the programme, apparently leaning toward the short take-off and vertical landing (STOVL) F-35B variant.

The F-X III requirement, originally for 60 aircraft, was hotly contested between the Lockheed aircraft, as well as the Boeing F-15 Silent Eagle and Eurofighter Typhoon. The F-15SE appeared to have secured the win last summer, but Seoul abruptly decided to change the terms of the requirement to favour a stealthy aircraft.